A refining element is generally formed with a pattern of bars and intermediate grooves. The bars and grooves are formed in different ways, depending on which fibrous material is to be worked and the degree of working desired and in the case of lignocellulosic material, the pulp quality which is desired. The bars generally have an upper surface and side surfaces such that longitudinal edges are formed between the upper surface and the respective side surface. The bars can, for example, be continuous or discontinuous and arranged in various patterns. The working of the fibrous material is substantially carried out by the bars of the refining elements. The refining gap is formed so that the fibrous material, as seen in the radial direction, passes from the inside outwardly. Farthest inwardly in the refining gap, the refining elements are normally formed to bring about a first disintegration of the material and to advance the material outwardly in the refining gap. A certain defibering, i.e. separation of the fibers of the lignocellulosic material, also takes place in the inner portion of the refining gap, where the distance between the refining surfaces is the greatest. Thereafter this distance decreases outwardly in order that the desired working or refining of the fibrous material can be achieved.
During the refining of fibrous material of high concentration and, above all, at high energy inputs, it has been found necessary to form the outer portion of the refining element with a tight pattern of bars and grooves in order to thereby improve the access to the fibrous material and to bring about an effective working thereof. The bar width in that case can be from about 1 to 2 mm and the groove width can be from about 1.5 to 2 mm. This working at the same time generates a great amount of steam in the refining gap. This gives rise to a high steam pressure in the refining gap. This high steam pressure has a negative effect on the capacity and operational stability of the refiner. This also implies a restriction on the potential energy input. The steam which is developed will, as a result of the tight pattern, be forced up out of the grooves, and will disturb the material flow through the refining gap.
One way of solving this problem would be to supply dilution water to the refining gap in order to thereby condense the steam. This, however, would reduce the material concentration to a low level, and thereby deteriorate the pulp quality.
During the working or refining of fibrous material having a low concentration no steam development takes place, and the material is transported, partly by the liquid flow, out of the refining gap. In this case, a tight pattern of bars and grooves results in the flow through the refining gap being much too low.